This invention relates to the production of carbon fiber.
Carbon fibers may be produced from a variety of feedstocks. For example, rayon, polyacrylonitrile and isotropic and liquid crystalline pitches may be spun into fiber form and then carbonized at a temperature of about 800.degree. to 1000.degree. C. to produce a carbon fiber. The carbon fiber may thereafter be heat treated at a temperature of 2000.degree. to 3000.degree. C. to graphitize the fiber. This has the effect of increasing the fiber modulus. Production of carbon fibers from rayon and polyacrylonitrile has the disadvantage that the feedstock material is relatively expensive and low carbon conversion to fiber is achieved. While the use of isotropic pitch involves a less expensive feedstock, melt-spinning is necessary and carbon conversion is relatively low. Preparation of liquid crystalline pitch is expensive and melt-spinning is again necessary.
U.S. Pat. No. 3,461,082 describes a method of producing carbon fiber from lignin using melt-spinning, dry-spinning or wet-spinning processes. Conversion of the carbon in the lignin to carbon fiber is relatively low.
U.S. Pat. No. 3,723,609 describes a method of producing carbon fibers from various raw materials such as lignin, together with various film-forming polymers. In one example, an ammonium lignin sulphonate solution is mixed with an aqueous polyethylene oxide solution. The solution is homogenised with the introduction of ammonia gas up to a pH of 10. The solution is filtered and then spun to produce fibers which are taken up on a rotating drum. The fibers are dried and then carbonized to produce flexible carbon fibers. The use of lignin leads to low carbon conversion to fiber.
M. A. A. Jorro and W. R. Ladner, Carbon Fiber from Coal, Proceedings of the Fourth London International Carbon and Graphite Conference, September 1974, pages 287 to 303, describes the production of carbon fibers from a coal extract. The extract is produced by digesting the coal at elevated temperature and pressure with a high boiling aromatic solvent. The resulting mixture, in which the coal has been chemically modified, is filtered to remove the mineral matter and undissolved coal and then the bulk of the solvent is removed by distillation under reduced pressure. Some of the solvent is retained in the extract to make it suitable for spinning. The production of the carbonized fiber from the coal extract involves extrusion and drawing of the molten extract to filaments, heating the filaments in an oxidizing atmosphere at an increasing temperature up to 250.degree. C. to render them infusible and heating the filaments in an inert atmosphere up to a temperature of 1000.degree. C. to carbonize them. The process described is a melt-spinning process.